Name
ARB_shader_image_load_store
Name Strings
GL_ARB_shader_image_load_store
Contact
Jeff Bolz, NVIDIA Corporation (jbolz 'at' nvidia.com)
Pat Brown, NVIDIA Corporation (pbrown 'at' nvidia.com)
Contributors
Barthold Lichtenbelt, NVIDIA
Bill Licea-Kane, AMD
Eric Werness, NVIDIA
Graham Sellers, AMD
Greg Roth, NVIDIA
Nick Haemel, AMD
Pierre Boudier, AMD
Piers Daniell, NVIDIA
Notice
Copyright (c) 2011-2014 The Khronos Group Inc. Copyright terms at
http://www.khronos.org/registry/speccopyright.html
Status
Complete. Approved by the ARB on 2011/06/20.
Approved by the Khronos Promoters on 2011/07/29.
Version
Last Modified Date: September 11, 2014
Revision: 35
Number
ARB Extension #115
Dependencies
This extension is written against the OpenGL 3.2 specification
(Compatibility Profile).
This extension is written against version 1.50 (revision 09) of the OpenGL
Shading Language Specification.
OpenGL 3.0 and GLSL 1.30 are required.
This extension interacts trivially with OpenGL 3.2 (Core Profile).
This extension interacts trivially with OpenGL 3.1,
ARB_uniform_buffer_object, and EXT_bindable_uniform.
This extension interacts trivially with ARB_draw_indirect.
This extension interacts trivially with NV_vertex_buffer_unified_memory.
This extension interacts with NV_parameter_buffer_object.
This extension interacts trivially with OpenGL 3.2 and
ARB_texture_multisample.
This extension interacts trivially with OpenGL 4.0 and ARB_sample_shading.
This extension interacts trivially with OpenGL 4.0 and
ARB_texture_cube_map_array.
This extension interacts trivially with OpenGL 3.3 and
ARB_texture_rgb10_a2ui.
This extension interacts trivially with NV_shader_buffer_load.
This extension interacts trivially with OpenGL 4.0, ARB_gpu_shader5, and
NV_gpu_shader5.
This extension interacts trivially with OpenGL 4.0 and
ARB_tessellation_shader.
This extension interacts trivially with EXT_depth_bounds_test.
This extension interacts with ARB_separate_shader_objects.
This extension interacts with EXT_shader_image_load_store.
Overview
This extension provides GLSL built-in functions allowing shaders to load
from, store to, and perform atomic read-modify-write operations to a
single level of a texture object from any shader stage. These built-in
functions are named imageLoad(), imageStore(), and imageAtomic*(),
respectively, and accept integer texel coordinates to identify the texel
accessed. The extension adds the notion of "image units" to the OpenGL
API, to which texture levels are bound for access by the GLSL built-in
functions. To allow shaders to specify the image unit to access, GLSL
provides a new set of data types ("image*") similar to samplers. Each
image variable is assigned an integer value to identify an image unit to
access, which is specified using Uniform*() APIs in a manner similar to
samplers.
This extension also provides the capability to explicitly enable "early"
per-fragment tests, where operations like depth and stencil testing are
performed prior to fragment shader execution. In unextended OpenGL,
fragment shaders never have any side effects and implementations can
sometimes perform per-fragment tests and discard some fragments prior to
executing the fragment shader. Since this extension allows fragment
shaders to write to texture and buffer object memory using the built-in
image functions, such optimizations could lead to non-deterministic
results. To avoid this, implementations supporting this extension may not
perform such optimizations on shaders having such side effects. However,
enabling early per-fragment tests guarantees that such tests will be
performed prior to fragment shader execution, and ensures that image
stores and atomics will not be performed by fragment shader invocations
where these per-fragment tests fail.
Finally, this extension provides both a GLSL built-in function and an
OpenGL API function allowing applications some control over the ordering
of image loads, stores, and atomics relative to other OpenGL pipeline
operations accessing the same memory. Because the extension provides the
ability to perform random accesses to texture or buffer object memory,
such accesses are not easily tracked by the OpenGL driver. To avoid the
need for heavy-handed synchronization at the driver level, this extension
requires manual synchronization. The MemoryBarrier() OpenGL API
function allows applications to specify a bitfield indicating the set of
OpenGL API operations to synchronize relative to shader memory access.
The memoryBarrier() GLSL built-in function provides a synchronization
point within a given shader invocation to ensure that all memory accesses
performed prior to the synchronization point complete prior to any started
after the synchronization point.
New Procedures and Functions
void BindImageTexture(uint unit, uint texture, int level,
boolean layered, int layer, enum access,
enum format);
void MemoryBarrier(bitfield barriers);
New Tokens
Accepted by the parameter of GetBooleanv, GetIntegerv,
GetFloatv, GetDoublev, and GetInteger64v:
MAX_IMAGE_UNITS 0x8F38
MAX_COMBINED_IMAGE_UNITS_AND_FRAGMENT_OUTPUTS 0x8F39
MAX_IMAGE_SAMPLES 0x906D
MAX_VERTEX_IMAGE_UNIFORMS 0x90CA
MAX_TESS_CONTROL_IMAGE_UNIFORMS 0x90CB
MAX_TESS_EVALUATION_IMAGE_UNIFORMS 0x90CC
MAX_GEOMETRY_IMAGE_UNIFORMS 0x90CD
MAX_FRAGMENT_IMAGE_UNIFORMS 0x90CE
MAX_COMBINED_IMAGE_UNIFORMS 0x90CF
Accepted by the parameter of GetIntegeri_v and GetBooleani_v:
IMAGE_BINDING_NAME 0x8F3A
IMAGE_BINDING_LEVEL 0x8F3B
IMAGE_BINDING_LAYERED 0x8F3C
IMAGE_BINDING_LAYER 0x8F3D
IMAGE_BINDING_ACCESS 0x8F3E
IMAGE_BINDING_FORMAT 0x906E
Accepted by the parameter of MemoryBarrier:
VERTEX_ATTRIB_ARRAY_BARRIER_BIT 0x00000001
ELEMENT_ARRAY_BARRIER_BIT 0x00000002
UNIFORM_BARRIER_BIT 0x00000004
TEXTURE_FETCH_BARRIER_BIT 0x00000008
SHADER_IMAGE_ACCESS_BARRIER_BIT 0x00000020
COMMAND_BARRIER_BIT 0x00000040
PIXEL_BUFFER_BARRIER_BIT 0x00000080
TEXTURE_UPDATE_BARRIER_BIT 0x00000100
BUFFER_UPDATE_BARRIER_BIT 0x00000200
FRAMEBUFFER_BARRIER_BIT 0x00000400
TRANSFORM_FEEDBACK_BARRIER_BIT 0x00000800
ATOMIC_COUNTER_BARRIER_BIT 0x00001000
ALL_BARRIER_BITS 0xFFFFFFFF
Returned by the parameter of GetActiveUniform:
IMAGE_1D 0x904C
IMAGE_2D 0x904D
IMAGE_3D 0x904E
IMAGE_2D_RECT 0x904F
IMAGE_CUBE 0x9050
IMAGE_BUFFER 0x9051
IMAGE_1D_ARRAY 0x9052
IMAGE_2D_ARRAY 0x9053
IMAGE_CUBE_MAP_ARRAY 0x9054
IMAGE_2D_MULTISAMPLE 0x9055
IMAGE_2D_MULTISAMPLE_ARRAY 0x9056
INT_IMAGE_1D 0x9057
INT_IMAGE_2D 0x9058
INT_IMAGE_3D 0x9059
INT_IMAGE_2D_RECT 0x905A
INT_IMAGE_CUBE 0x905B
INT_IMAGE_BUFFER 0x905C
INT_IMAGE_1D_ARRAY 0x905D
INT_IMAGE_2D_ARRAY 0x905E
INT_IMAGE_CUBE_MAP_ARRAY 0x905F
INT_IMAGE_2D_MULTISAMPLE 0x9060
INT_IMAGE_2D_MULTISAMPLE_ARRAY 0x9061
UNSIGNED_INT_IMAGE_1D 0x9062
UNSIGNED_INT_IMAGE_2D 0x9063
UNSIGNED_INT_IMAGE_3D 0x9064
UNSIGNED_INT_IMAGE_2D_RECT 0x9065
UNSIGNED_INT_IMAGE_CUBE 0x9066
UNSIGNED_INT_IMAGE_BUFFER 0x9067
UNSIGNED_INT_IMAGE_1D_ARRAY 0x9068
UNSIGNED_INT_IMAGE_2D_ARRAY 0x9069
UNSIGNED_INT_IMAGE_CUBE_MAP_ARRAY 0x906A
UNSIGNED_INT_IMAGE_2D_MULTISAMPLE 0x906B
UNSIGNED_INT_IMAGE_2D_MULTISAMPLE_ARRAY 0x906C
Accepted by the parameter of GetTexParameteriv, GetTexParameterfv,
GetTexParameterIiv, and GetTexParameterIuiv:
IMAGE_FORMAT_COMPATIBILITY_TYPE 0x90C7
Returned in the parameter of GetTexParameteriv, GetTexParameterfv,
GetTexParameterIiv, and GetTexParameterIuiv when is
IMAGE_FORMAT_COMPATIBILITY_TYPE:
IMAGE_FORMAT_COMPATIBILITY_BY_SIZE 0x90C8
IMAGE_FORMAT_COMPATIBILITY_BY_CLASS 0x90C9
Additions to Chapter 2 of the OpenGL 3.2 (Compatibility Profile) Specification
(Rasterization)
Modify Section 2.14.4, Uniform Variables, p. 89
(modify second paragraph, p. 90) Sets of uniforms, except for samplers
and images, can be grouped into uniform blocks. ...
(Add new types to table 2.13, pp. 96-98)
Type Name Keyword
------------------------------ -------------------------
IMAGE_1D image1D
IMAGE_2D image2D
IMAGE_3D image3D
IMAGE_2D_RECT image2DRect
IMAGE_CUBE imageCube
IMAGE_BUFFER imageBuffer
IMAGE_1D_ARRAY image1DArray
IMAGE_2D_ARRAY image2DArray
IMAGE_CUBE_MAP_ARRAY imageCubeArray
IMAGE_2D_MULTISAMPLE image2DMS
IMAGE_2D_MULTISAMPLE_ARRAY image2DMSArray
INT_IMAGE_1D iimage1D
INT_IMAGE_2D iimage2D
INT_IMAGE_3D iimage3D
INT_IMAGE_2D_RECT iimage2DRect
INT_IMAGE_CUBE iimageCube
INT_IMAGE_BUFFER iimageBuffer
INT_IMAGE_1D_ARRAY iimage1DArray
INT_IMAGE_2D_ARRAY iimage2DArray
INT_IMAGE_CUBE_MAP_ARRAY iimageCubeArray
INT_IMAGE_2D_MULTISAMPLE iimage2DMS
INT_IMAGE_2D_MULTISAMPLE_ARRAY iimage2DMSArray
UNSIGNED_INT_IMAGE_1D uimage1D
UNSIGNED_INT_IMAGE_2D uimage2D
UNSIGNED_INT_IMAGE_3D uimage3D
UNSIGNED_INT_IMAGE_2D_RECT uimage2DRect
UNSIGNED_INT_IMAGE_CUBE uimageCube
UNSIGNED_INT_IMAGE_BUFFER uimageBuffer
UNSIGNED_INT_IMAGE_1D_ARRAY uimage1DArray
UNSIGNED_INT_IMAGE_2D_ARRAY uimage2DArray
UNSIGNED_INT_IMAGE_CUBE_MAP_ARRAY uimageCubeArray
UNSIGNED_INT_IMAGE_2D_MULTISAMPLE uimage2DMS
UNSIGNED_INT_IMAGE_2D_MULTISAMPLE_ARRAY uimage2DMSArray
(Add a new subsection after Section 2.14.5, Samplers, p. 106)
Section 2.14.X, Images
Images are special uniforms used in the OpenGL Shading Language to
identify a level of a texture to be read or written using image load,
store, and atomic built-in functions in the manner described in Section
3.9.X. The value of an image uniform is an integer specifying the image
unit accessed. Image units are numbered beginning at zero, and there is
an implementation-dependent number of available image units
(MAX_IMAGE_UNITS). The error INVALID_VALUE is generated if a
Uniform1i{v} call is used to set an image uniform to a value less than
zero or greater than or equal to MAX_IMAGE_UNITS. Note that image
units used for image variables are independent of the texture image
units used for sampler variables; the number of units provided by the
implementation may differ. Textures are bound independently and
separately to image and texture image units.
The type of an image variable must match the texture target of the image
currently bound to the image unit, otherwise the result of a load, store,
or atomic operation is undefined (see Section 4.1.X of the OpenGL
Shading Language specification for more detail).
The location of an image variable needs to be queried with
GetUniformLocation, just like any uniform variable. Image values need to
be set by calling Uniform1i{v}. Loading image variables with any of the
other Uniform entry point is not allowed and will result in an
INVALID_OPERATION error.
Unlike samplers, there is no limit on the number of active image variables
that may be used by a program or by any particular shader. However, given
that there is an implementation-dependent limit on the number of unique
image units, the actual number of images that may be used by all shaders
in a program is limited.
Modify Section 2.14.7, Shader Execution, p. 109
(Add a new unnumbered subsection before "Shader Inputs", p. 113)
Image Access
Shaders have the ability read and write to textures using image uniforms.
The maximum number of image uniforms available to individual shader stages
are the values of the implementation dependent constants
* MAX_VERTEX_IMAGE_UNIFORMS (vertex shaders),
* MAX_TESS_CONTROL_IMAGE_UNIFORMS (tessellation control shaders),
* MAX_TESS_EVALUATION_IMAGE_UNIFORMS (tessellation evaluation shaders),
* MAX_GEOMETRY_IMAGE_UNIFORMS (geometry shaders), and
* MAX_FRAGMENT_IMAGE_UNIFORMS (fragment shaders).
All active shaders combined cannot use more than the value of
MAX_COMBINED_IMAGE_UNIFORMS atomic counters. If more than one shader stage
accesses the same image uniform, each such access counts separately
against the MAX_COMBINED_IMAGE_UNIFORMS limit.
(Add a new numbered subsection after Section 2.14.7, Shader Execution,
p. 109)
Section 2.14.X, Shader Memory Access
Shaders may perform random-access reads and writes to texture or buffer
object memory using built-in image load, store, and atomic functions, as
described in the OpenGL Shading Language Specification. The ability to
perform such random-access reads and writes in systems that may be highly
pipelined results in ordering and synchronization issues discussed in the
sections below.
Shader Memory Access Ordering
The order in which texture or buffer object memory is read or written by
shaders is largely undefined. For some shader types (vertex, tessellation
evaluation, and in some cases, fragment), even the number of shader
invocations that might perform loads and stores is undefined.
In particular, the following rules apply:
* While a vertex or tessellation evaluation shader will be executed at
least once for each unique vertex specified by the application (vertex
shaders) or generated by the tessellation primitive generator
(tessellation evaluation shaders), it may be executed more than once
for implementation-dependent reasons. Additionally, if the same
vertex is specified multiple times in a collection of primitives
(e.g., repeating an index in DrawElements), the vertex shader might be
run only once.
* For each fragment generated by the GL, the number of fragment shader
invocations depends on a number of factors. If the fragment fails the
pixel ownership test (Section 4.1.1), the fragment shader may not be
executed. Otherwise, if the framebuffer has no multisample buffer
(SAMPLE_BUFFERS is zero), the fragment shader will be invoked exactly
once. If the fragment shader specifies per-sample shading, the
fragment shader will be run once per covered sample. Otherwise, the
number of fragment shader invocations is undefined, but must be in the
range [1,], where is the number of samples covered by the
fragment.
* If a fragment shader is invoked to process fragments or samples not
covered by a primitive being rasterized to facilitate the
approximation of derivatives for texture lookups, stores and atomics
have no effect.
* The relative order of invocations of the same shader type are
undefined. A store issued by a shader when working on primitive B
might complete prior to a store for primitive A, even if primitive A
is specified prior to primitive B. This applies even to fragment
shaders; while fragment shader outputs are written to the framebuffer
in primitive order, stores executed by fragment shader invocations are
not.
* The relative order of invocations of different shader types is largely
undefined. However, when executing a shader whose inputs are
generated from a previous programmable stage, the shader invocations
from the previous stage are guaranteed to have executed far enough to
generate final values for all next-stage inputs. That implies shader
completion for all stages except geometry; geometry shaders are
guaranteed only to have executed far enough to emit all needed
vertices.
The above limitations on shader invocation order also make some forms of
synchronization between shader invocations within a single set of
primitives unimplementable. For example, having one invocation poll
memory written by another invocation assumes that the other invocation has
been launched and can complete its writes. The only case where such a
guarantee is made is when the inputs of one shader invocation are
generated from the outputs of a shader invocation in a previous stage.
Stores issued to different memory locations within a single shader
invocation may not be visible to other invocations in the order they were
performed. The built-in function memoryBarrier() may be used to provide
stronger ordering of reads and writes performed by a single invocation.
Calling memoryBarrier() guarantees that any memory transactions issued by
the shader invocation prior to the call complete prior to the memory
transactions issued after the call. Memory barriers may be needed for
algorithms that require multiple invocations to access the same memory and
require the operations need to be performed in a partially-defined
relative order. For example, if one shader invocation does a series of
writes, followed by a memoryBarrier() call, followed by another write,
then another invocation that sees the results of the final write will also
see the previous writes. Without the memory barrier, the final write may
be visible before the previous writes.
The atomic memory transaction built-in functions may be used to read and
write a given memory address atomically. While atomic built-in functions
issued by multiple shader invocations are executed in undefined order
relative to each other, these functions perform both a read and a write of
a memory address and guarantee that no other memory transaction will write
to the underlying memory between the read and write. Atomics allow
shaders to use shared global addresses for mutual exclusion or as
counters, among other uses.
Shader Memory Access Synchronization
Data written to textures or buffer objects by a shader invocation may
eventually be read by other shader invocations, sourced by other fixed
pipeline stages, or read back by the application. When applications write
to buffer objects or textures using API commands such as TexSubImage* or
BufferSubData, the GL implementation knows when and where writes occur and
can perform implicit synchronization to ensure that operations requested
before the update see the original data and that subsequent operations see
the modified data. Without logic to track the target address of each
shader instruction performing a store, automatic synchronization of stores
performed by a shader invocation would require the GL implementation to
make worst-case assumptions at significant performance cost. To permit
cases where textures or buffers may be read or written in different
pipeline stages without the overhead of automatic synchronization, buffer
object and texture stores performed by shaders are not automatically
synchronized with other GL operations using the same memory.
Explicit synchronization is required to ensure that the effects of buffer
and texture data stores performed by shaders will be visible to subsequent
operations using the same objects and will not overwrite data still to be
read by previously requested operations. Without manual synchronization,
shader stores for a "new" primitive may complete before processing of an
"old" primitive completes. Additionally, stores for an "old" primitive
might not be completed before processing of a "new" primitive starts. The
command
void MemoryBarrier(bitfield barriers)
defines a barrier ordering the memory transactions issued prior to the
command relative to those issued after the barrier. For the purposes of
this ordering, memory transactions performed by shaders are considered to
be issued by the rendering command that triggered the execution of the
shader. is a bitfield indicating the set of operations that
are synchronized with shader stores; the bits used in are as
follows:
- VERTEX_ATTRIB_ARRAY_BARRIER_BIT: If set, vertex data sourced from
buffer objects after the barrier will reflect data written by shaders
prior to the barrier. The set of buffer objects affected by this bit
is derived from the buffer object bindings or GPU addresses used for
generic vertex attributes (VERTEX_ATTRIB_ARRAY_BUFFER bindings,
VERTEX_ATTRIB_ARRAY_ADDRESS from NV_vertex_buffer_unified_memory), as
well as those for arrays of named vertex attributes (e.g., vertex,
color, normal).
- ELEMENT_ARRAY_BARRIER_BIT: If set, vertex array indices sourced from
buffer objects after the barrier will reflect data written by shaders
prior to the barrier. The buffer objects affected by this bit are
derived from the ELEMENT_ARRAY_BUFFER binding and the
NV_vertex_buffer_unified_memory ELEMENT_ARRAY_ADDRESS address.
- UNIFORM_BARRIER_BIT: Shader uniforms and assembly program parameters
sourced from buffer objects after the barrier will reflect data
written by shaders prior to the barrier.
- TEXTURE_FETCH_BARRIER_BIT: Texture fetches from shaders, including
fetches from buffer object memory via buffer textures, after the
barrier will reflect data written by shaders prior to the barrier.
- SHADER_IMAGE_ACCESS_BARRIER_BIT: Memory accesses using shader image
load, store, and atomic built-in functions issued after the barrier
will reflect data written by shaders prior to the barrier.
Additionally, image stores and atomics issued after the barrier will
not execute until all memory accesses (e.g., loads, stores, texture
fetches, vertex fetches) initiated prior to the barrier complete.
- COMMAND_BARRIER_BIT: Command data sourced from buffer objects by
Draw*Indirect commands after the barrier will reflect data written by
shaders prior to the barrier. The buffer objects affected by this bit
are derived from the DRAW_INDIRECT_BUFFER binding and the GPU
address DRAW_INDIRECT_ADDRESS_NV.
- PIXEL_BUFFER_BARRIER_BIT: Reads/writes of buffer objects via the
PACK/UNPACK_BUFFER bindings (ReadPixels, TexSubImage, etc.) after the
barrier will reflect data written by shaders prior to the barrier.
Additionally, buffer object writes issued after the barrier will wait
on the completion of all shader writes initiated prior to the barrier.
- TEXTURE_UPDATE_BARRIER_BIT: Writes to a texture via Tex(Sub)Image*,
CopyTex(Sub)Image*, CompressedTex(Sub)Image*, and reads via
GetTexImage after the barrier will reflect data written by shaders
prior to the barrier. Additionally, texture writes from these
commands issued after the barrier will not execute until all shader
writes initiated prior to the barrier complete.
- BUFFER_UPDATE_BARRIER_BIT: Reads/writes via Buffer(Sub)Data,
CopyBufferSubData, ProgramBufferParametersNV, and GetBufferSubData, or
to buffer object memory mapped by MapBuffer(Range) after the barrier
will reflect data written by shaders prior to the barrier.
Additionally, writes via these commands issued after the barrier will
wait on the completion of any shader writes to the same memory
initiated prior to the barrier.
- FRAMEBUFFER_BARRIER_BIT: Reads and writes via framebuffer object
attachments after the barrier will reflect data written by shaders
prior to the barrier. Additionally, framebuffer writes issued after
the barrier will wait on the completion of all shader writes issued
prior to the barrier.
- TRANSFORM_FEEDBACK_BARRIER_BIT: Writes via transform feedback
bindings after the barrier will reflect data written by shaders prior
to the barrier. Additionally, transform feedback writes issued after
the barrier will wait on the completion of all shader writes issued
prior to the barrier.
- ATOMIC_COUNTER_BARRIER_BIT: Accesses to atomic counters after the
barrier will reflect writes prior to the barrier.
If is ALL_BARRIER_BITS, shader memory accesses will be
synchronized relative to all the operations described above.
Implementations may cache buffer object and texture image memory that
could be written by shaders in multiple caches; for example, there may be
separate caches for texture, vertex fetching, and one or more caches for
shader memory accesses. Implementations are not required to keep these
caches coherent with shader memory writes. Stores issued by one
invocation may not be immediately observable by other pipeline stages or
other shader invocations because the value stored may remain in a cache
local to the processor executing the store, or because data overwritten by
the store is still in a cache elsewhere in the system. When MemoryBarrier
is called, the GL flushes and/or invalidates any caches relevant to the
operations specified by the parameter to ensure consistent
ordering of operations across the barrier.
To allow for independent shader invocations to communicate by reads and
writes to a common memory address, image variables in the OpenGL Shading
Language may be declared as "coherent". Buffer object or texture image
memory accessed through such variables may be cached only if caches are
automatically updated due to stores issued by any other shader invocation.
If the same address is accessed using both coherent and non-coherent
variables, the accesses using variables declared as coherent will observe
the results stored using coherent variables in other invocations. Using
variables declared as "coherent" guarantees only that the results of
stores will be immediately visible to shader invocations using
similarly-declared variables; calling MemoryBarrier is required to ensure
that the stores are visible to other operations.
The following guidelines may be helpful in choosing when to use coherent
memory accesses and when to use barriers.
- Data that are read-only or constant may be accessed without using
coherent variables or calling MemoryBarrier(). Updates to the
read-only data via API calls such as BufferSubData will invalidate
shader caches implicitly as required.
- Data that are shared between shader invocations at a fine granularity
(e.g., written by one invocation, consumed by another invocation) should
use coherent variables to read and write the shared data.
- Data written by one shader invocation and consumed by other shader
invocations launched as a result of its execution ("dependent
invocations") should use coherent variables in the producing shader
invocation and call memoryBarrier() after the last write. The consuming
shader invocation should also use coherent variables.
- Data written to image variables in one rendering pass and read by the
shader in a later pass need not use coherent variables or
memoryBarrier(). Calling MemoryBarrier() with the
SHADER_IMAGE_ACCESS_BARRIER_BIT set in between passes is
necessary.
- Data written by the shader in one rendering pass and read by another
mechanism (e.g., vertex or index buffer pulling) in a later pass need
not use coherent variables or memoryBarrier(). Calling
MemoryBarrier() with the appropriate bits set in between
passes is necessary.
Additions to Chapter 3 of the OpenGL 3.2 (Compatibility Profile) Specification
(Rasterization)
(insert new section immediately before Section 3.8, Texturing, p. 210)
Section 3.X, Early Per-Fragment Tests
Once fragments are produced by rasterization (sections 3.4 through 3.8), a
number of per-fragment operations may be performed prior to fragment
shader execution. If a fragment is discarded during any of these
operations, it will not be processed by any subsequent stage, including
fragment shader execution.
Up to six operations are performed on each fragment, in the following
order:
* the pixel ownership test, described in section 4.1.1;
* the scissor test, described in section 4.1.2;
* the depth bounds test, described in section 4.1.X (of the
EXT_depth_bounds_test specification);
* the stencil test, described in section 4.1.5;
* the depth buffer test, described in section 4.1.6; and
* occlusion query sample counting, described in section 4.1.7.
The pixel ownership and scissor tests are always performed.
The other operations are performed if and only if early fragment tests are
enabled in the active fragment shader (section 3.12.2). When early
per-fragment operations are enabled, the depth bounds test, stencil test,
depth buffer test, and occlusion query sample counting operations are
performed prior to fragment shader execution, and the stencil buffer,
depth buffer, and occlusion query sample counts will be updated
accordingly. When early per-fragment operations are enabled, these
operations will not be performed again after fragment shader execution.
When there is no active program, the active program has no fragment
shader, or the active program was linked with early fragment tests
disabled, these operations are performed only after fragment program
execution, in the order described in chapter 4.
If early fragment tests are enabled, any depth value computed by the
fragment shader has no effect. Additionally, the depth buffer, stencil
buffer, and occlusion query sample counts may be updated even for
fragments or samples that would be discarded after fragment shader
execution due to per-fragment operations such as alpha-to-coverage or
alpha tests.
(Add new section after Section 3.9.19, Texture Application, p. 268)
Section 3.9.X, Texture Image Loads and Stores
The contents of a texture may be made available for shaders to read and
write by binding the texture to one of a collection of image units. The
GL implementation provides an array of image units numbered beginning with
zero, with the total number of image units provided given by the
implementation-dependent constant MAX_IMAGE_UNITS. Unlike texture image
units, image units do not have a separate attachment for each texture
target texture; each image unit may have only one texture bound at a time.
A texture may be bound to an image unit for use by image loads and stores
by calling:
void BindImageTexture(uint unit, uint texture, int level,
boolean layered, int layer, enum access,
enum format);
where identifies the image unit, is the name of the
texture, and selects a single level of the texture. If
is zero, any texture currently bound to image unit is unbound. If
is greater than or equal to the value of MAX_IMAGE_UNITS, if
or is less than zero, or if is not the name of
an existing texture object, the error INVALID_VALUE is generated.
If the texture identified by is a one-dimensional array,
two-dimensional array, three-dimensional, cube map, cube map array, or
two-dimensional multisample array texture, it is possible to bind either
the entire texture level or a single layer or face of the texture level.
If is TRUE, the entire level is bound. If is FALSE,
only the single layer identified by will be bound. When
is FALSE, the single bound layer is treated as a different texture target
for image accesses:
* one-dimensional array texture layers are treated as one-dimensional
textures;
* two-dimensional array, three-dimensional, cube map, cube map array
texture layers are treated as two-dimensional textures; and
* two-dimensional multisample array textures are treated as
two-dimensional multisample textures.
For cube map textures where is FALSE, the face is taken by
mapping the layer number to a face according to table 4.13. For cube map
array textures where is FALSE, the selected layer number is
mapped to a texture layer and cube face using the following equations and
mapping to a face according to table 4.13.
layer = floor(layer_orig / 6)
face = layer_orig - (layer * 6)
If the texture identified by does not have multiple layers or
faces, the entire texture level is bound, regardless of the values
specified by and .
specifies the format that the elements of the image will be
treated as when doing formatted stores, as described later in this
section. This is referred to as the "image unit format". This must be one
of the formats listed in Table X.2; otherwise, the error INVALID_VALUE is
generated.
specifies whether the texture bound to the image will be treated
as READ_ONLY, WRITE_ONLY, or READ_WRITE. If a shader reads from an image
unit with a texture bound as WRITE_ONLY, or writes to an image unit with a
texture bound as READ_ONLY, the results of that shader operation are
undefined and may lead to application termination.
If a texture object bound to one or more image units is deleted by
DeleteTextures, it is detached from each such image unit, as though
BindImageTexture were called with identifying the image unit and
set to zero.
When a shader accesses the texture bound to an image unit using a built-in
image load, store, or atomic function, it identifies a single texel by
providing a one-, two-, or three-dimensional coordinate. Multisample
texture accesses also specify a sample number. A coordinate vector is
mapped to an individual texel tau_i, tau_i_j, or tau_i_j_k according to
the target of the texture bound to the image unit using Table X.1. As
noted above, single-layer bindings of array or cube map textures are
considered to use a texture target corresponding to the bound layer,
rather than that of the full texture.
Face/
i j k layer
-- -- -- -----
TEXTURE_1D x - - -
TEXTURE_2D x y - -
TEXTURE_3D x y z -
TEXTURE_RECTANGLE x y - -
TEXTURE_CUBE_MAP x y - z
TEXTURE_BUFFER x - - -
TEXTURE_1D_ARRAY x - - y
TEXTURE_2D_ARRAY x y - z
TEXTURE_CUBE_MAP_ARRAY x y - z
TEXTURE_2D_MULTISAMPLE x y - -
TEXTURE_2D_MULTISAMPLE_ARRAY x y - z
Table X.1, Mapping of image load, store, and atomic texel coordinate
components to texel numbers.
If the texture target has layers or cube map faces, the layer or face
number is taken from the argument of BindImageTexture if the
texture is bound with set to FALSE, or from the coordinate
identified by Table X.1 otherwise. For cube map and cube map array
textures with set to TRUE, the coordinate is mapped to a layer
and face in the same manner as described for the argument of
BindImageTexture.
If the individual texel identified for an image load, store, or atomic
operation doesn't exist, the access is treated as invalid. Invalid image
loads will return zero. Invalid image stores will have no effect.
Invalid image atomics will not update any texture bound to the image unit
and will return zero. An access is considered invalid if:
* no texture is bound to the selected image unit;
* the texture bound to the selected image unit is incomplete;
* the texture level bound to the image unit is less than the base
level or greater than the maximum level of the texture;
* [[compatiblity profile only]] the texture bound to the image unit is
bordered;
* the internal format of the texture bound to the image unit is not
found in Table X.2;
* the internal format of the texture bound to the image unit is
incompatible with the specified according to Table X.3;
* the texture bound to the image unit has layers, and the selected layer
or cube map face doesn't exist;
* the selected texel tau_i, tau_i_j, or tau_i_j_k doesn't exist;
* the image has more samples than the implementation-dependent value of
MAX_IMAGE_SAMPLES.
Additionally, there are a number of cases where image load, store, or
atomic operations are considered to involve a format mismatch. In such
cases, undefined values will be returned by image loads and atomic
operations and undefined values will be written by stores and atomic
operations. A format mismatch will occur if:
* the type of image variable used to access the image unit does not
match the target of a texture bound to the image unit with
set to TRUE;
* the type of image variable used to access the image unit does not
match the target corresponding to a single layer of a multi-layer
texture target bound to the image unit with set to FALSE;
* the type of image variable used to access the image unit has a
component data type (floating-point, signed integer, unsigned integer)
incompatible with the format of the image unit;
* the format layout qualifier for an image variable used for an image
load or atomic operation does not match the format of the image unit,
according to Table X.2; or
* the image variable used for an image store has a format layout
qualifier, and that qualifier does not match the format of the image
unit, according to Table X.2.
For textures with multiple samples per texel, the sample selected for an
image load, store, or atomic is undefined if the coordinate is
negative or greater than or equal to the number of samples in the
texture.
If a shader performs an image load, store, or atomic operation using an
image variable declared as an array, and if the index used to select an
individual element is negative or greater than or equal to the size
of the array, the results of the operation are undefined but may not lead
to termination.
Accesses to textures bound to image units do format conversions based on
the argument specified when the image is bound. Loads always
return a value as a vec4, ivec4, or uvec4, and stores always take the
source data as a vec4, ivec4, or uvec4. Data are converted to/from the
specified format according to the process described for a TexImage2D or
GetTexImage command with and as RGBA and FLOAT for vec4
data, with and as RGBA_INTEGER and INT for ivec4 data, or
with and as RGBA_INTEGER and UNSIGNED_INT for uvec4 data.
Unused components are filled in with (0,0,0,1) (where "1" is either a
floating-point or integer value, depending on the format).
Any image variable used for shader loads or atomic memory operations must
be declared with a format layout qualifier matching the format of its
associated image unit, as enumerated in Table X.2. Otherwise, the access
is considered to involve a format mismatch, as described above. Image
variables used exclusively for image stores need not include a format
layout qualifier, but any declared qualifier must match the image unit
format to avoid a format mismatch.
Image Unit Format Format Qualifer
----------------- ---------------
RGBA32F rgba32f
RGBA16F rgba16f
RG32F rg32f
RG16F rg16f
R11F_G11F_B10F r11f_g11f_b10f
R32F r32f
R16F r16f
RGBA32UI rgba32ui
RGBA16UI rgba16ui
RGB10_A2UI rgb10_a2ui
RGBA8UI rgba8ui
RG32UI rg32ui
RG16UI rg16ui
RG8UI rg8ui
R32UI r32ui
R16UI r16ui
R8UI r8ui
RGBA32I rgba32i
RGBA16I rgba16i
RGBA8I rgba8i
RG32I rg32i
RG16I rg16i
RG8I rg8i
R32I r32i
R16I r16i
R8I r8i
RGBA16 rgba16
RGB10_A2 rgb10_a2
RGBA8 rgba8
RG16 rg16
RG8 rg8
R16 r16
R8 r8
RGBA16_SNORM rgba16_snorm
RGBA8_SNORM rgba8_snorm
RG16_SNORM rg16_snorm
RG8_SNORM rg8_snorm
R16_SNORM r16_snorm
R8_SNORM r8_snorm
Table X.2, Supported image unit formats, with equivalent format
layout qualifiers.
When a texture is bound to an image unit, the parameter for the
image unit need not exactly match the texture internal format as long as
the formats are considered compatible. A pair of formats is considered
to match in size if the corresponding entries in the "size" column of
able X.3 are identical. A pair of formats is considered to match by
class if the corresponding entries in the "class" column of Table X.3 are
identical. For textures allocated by the GL, an image unit format is
compatible with a texture internal format if they match by size. For
textures allocated outside the GL, format compatibility is determined by
matching by size or by class, in an implementation dependent manner. The
matching criterion used for a given texture may be determined by calling
GetTexParameter with set to IMAGE_FORMAT_COMPATIBILITY_TYPE, with
return values of IMAGE_FORMAT_COMPATIBILITY_BY_SIZE and
IMAGE_FORMAT_COMPATIBILITY_BY_CLASS, specifying matches by size and
class, respectively.
When the format associated with an image unit does not exactly match the
internal format of the texture bound to the image unit, image loads,
stores, and atomic operations re-interpret the memory holding the
components of an accessed texel according to the format of the image unit.
The re-interpretation for image loads and the read portion of image
atomics is performed as though data were copied from the texel of the
bound texture to a similar texel represented in the format of the image
unit. Similarly, the re-interpretation for image stores and the write
portion of image atomics is performed as though data were copied from a
texel represented in the format of the image unit to the texel in the
bound texture. In both cases, this copy operation would be performed by:
* reading the texel from the source format to scratch memory according
to the process described for GetTexImage (section 6.1.4), using
default pixel storage modes and and parameters
corresponding to the source format in Table X.3; and
* writing the texel from scratch memory to the destination format
according to the process described for TexSubImage3D (section 3.9.2),
using default pixel storage modes and and parameters
corresponding to the destination format in Table X.3.
[[compatibility profile only: No pixel transfer operations are performed
during this conversion.]]
Image Format Size Class Pixel Format/Type
-------------- ---- ----- -----------------------------------------
RGBA32F 128 4x32 RGBA, FLOAT
RGBA16F 64 4x16 RGBA, HALF_FLOAT
RG32F 64 2x32 RG, FLOAT
RG16F 32 2x16 RG, HALF_FLOAT
R11F_G11F_B10F 32 (a) RGB, UNSIGNED_INT_10F_11F_11F_REV
R32F 32 1x32 RED, FLOAT
R16F 16 1x16 RED, HALF_FLOAT
RGBA32UI 128 4x32 RGBA_INTEGER, UNSIGNED_INT
RGBA16UI 64 4x16 RGBA_INTEGER, UNSIGNED_SHORT
RGB10_A2UI 32 (b) RGBA_INTEGER, UNSIGNED_INT_2_10_10_10_REV
RGBA8UI 32 4x8 RGBA_INTEGER, UNSIGNED_BYTE
RG32UI 64 2x32 RG_INTEGER, UNSIGNED_INT
RG16UI 32 2x16 RG_INTEGER, UNSIGNED_SHORT
RG8UI 16 2x8 RG_INTEGER, UNSIGNED_BYTE
R32UI 32 1x32 RED_INTEGER, UNSIGNED_INT
R16UI 16 1x16 RED_INTEGER, UNSIGNED_SHORT
R8UI 8 1x8 RED_INTEGER, UNSIGNED_BYTE
RGBA32I 128 4x32 RGBA_INTEGER, INT
RGBA16I 64 4x16 RGBA_INTEGER, SHORT
RGBA8I 32 4x8 RGBA_INTEGER, BYTE
RG32I 64 2x32 RG_INTEGER, INT
RG16I 32 2x16 RG_INTEGER, SHORT
RG8I 16 2x8 RG_INTEGER, BYTE
R32I 32 1x32 RED_INTEGER, INT
R16I 16 1x16 RED_INTEGER, SHORT
R8I 8 1x8 RED_INTEGER, BYTE
RGBA16 64 4x16 RGBA, UNSIGNED_SHORT
RGB10_A2 32 (b) RGBA, UNSIGNED_INT_2_10_10_10_REV
RGBA8 32 4x8 RGBA, UNSIGNED_BYTE
RG16 32 2x16 RG, UNSIGNED_SHORT
RG8 16 2x8 RG, UNSIGNED_BYTE
R16 16 1x16 RED, UNSIGNED_SHORT
R8 8 1x8 RED, UNSIGNED_BYTE
RGBA16_SNORM 64 4x16 RGBA, SHORT
RGBA8_SNORM 32 4x8 RGBA, BYTE
RG16_SNORM 32 2x16 RG, SHORT
RG8_SNORM 16 2x8 RG, BYTE
R16_SNORM 16 1x16 RED, SHORT
R8_SNORM 8 1x8 RED, BYTE
Table X.3, Texel sizes, compatibility classes, and pixel format/type
combinations for each image format. Class (a) is for 11/11/10 packed
floating-point formats; class (b) is for 10/10/10/2 packed formats.
Implementations may support a limited combined number of image units and
active fragment shader outputs (section 4.2.1). A link error will be
generated if the number of active image uniforms used in all shaders and
the number of active fragment shader outputs exceeds the implementation-
dependent value (MAX_COMBINED_IMAGE_UNITS_AND_FRAGMENT_OUTPUTS).
Modify Section 3.12.2, Shader Execution, p. 274
(add new unnumbered subsection section at the end of the section, p. 279)
Early Fragment Tests
An explicit control is provided to allow fragment shaders to enable early
fragment tests. If the fragment shader specifies the
"early_fragment_tests" layout qualifier, the per-fragment tests described
in Section 3.X will be performed prior to fragment shader execution.
Otherwise, they will be performed after fragment shader execution.
Additions to Chapter 4 of the OpenGL 3.2 (Compatibility Profile) Specification
(Per-Fragment Operations and the Framebuffer)
None.
Additions to Chapter 5 of the OpenGL 3.2 (Compatibility Profile) Specification
(Special Functions)
Modify Section 5.4.1, Commands Not Usable In Display Lists (p. 358)
(add "MemoryBarrier" to the list of commands not allowed in a display
list, in the "Buffer objects" paragraph)
Additions to Chapter 6 of the OpenGL 3.2 (Compatibility Profile) Specification
(State and State Requests)
Modify Section 6.1.3, Enumerated Queries (p. 369)
(modify 2nd pargraph, p. 370) ... must be TEXTURE_RESIDENT,
IMAGE_FORMAT_COMPATIBILITY_TYPE, or one of the symbolic values in table
3.22.
New Implementation Dependent State
Minimum
Get Value Type Get Command Value Description Sec. Attrib
--------- ---- ----------- -------- ----------------------- ---- ------
MAX_IMAGE_UNITS Z+ GetIntegerv 8 number of units for 3.9.X -
image load/store/atom
MAX_COMBINED_IMAGE_UNITS_ Z+ GetIntegerv 8 limit on active image 3.9.X -
AND_FRAGMENT_OUTPUTS units + fragment outputs
MAX_IMAGE_SAMPLES Z GetIntegerv 0 max allowed samples 3.9.X -
for a texture level
bound to an image unit
MAX_VERTEX_IMAGE_ Z+ GetIntegerv 0 number of image variables 2.14.7
UNIFORMS in vertex shaders
MAX_TESS_CONTROL_IMAGE_ Z+ GetIntegerv 0 number of image variables 2.14.7
UNIFORMS in tess. control shaders
MAX_TESS_EVALUATION_IMAGE_ Z+ GetIntegerv 0 number of image variables 2.14.7
UNIFORMS in tess. eval. shaders
MAX_GEOMETRY_IMAGE_ Z+ GetIntegerv 0 number of image variables 2.14.7
UNIFORMS in geometry shaders
MAX_FRAGMENT_IMAGE_ Z+ GetIntegerv 8 number of image variables 2.14.7
UNIFORMS in fragment shaders
MAX_COMBINED_IMAGE_ Z+ GetIntegerv 8 number of image variables 2.14.7
UNIFORMS in all shaders
New State
Add to Table 6.22, Textures (state per texture object), p. 414
Get Value Type Get Command Initial Value Description Sec Attribute
--------------------- ----- ----------- ------------- ------------------------ ----- ---------
IMAGE_FORMAT_ Z_2 GetTexParam- see 3.9.x compatibility rules for 3.9.X texture
COMPATIBILITY_TYPE eteriv texture use with image
units
Add a new Table 6.X, Image Stage (state per image unit)
Get Value Type Get Command Initial Value Description Sec Attribute
--------------------- ---- ----------- ------------- ------------------------ ----- ---------
IMAGE_BINDING_NAME 8*xZ+ GetIntegeri_v 0 name of bound texture 3.9.X none
object
IMAGE_BINDING_LEVEL 8*xZ+ GetIntegeri_v 0 level of bound texture 3.9.X none
object
IMAGE_BINDING_LAYERED 8*xB GetBooleani_v FALSE texture object bound w/ 3.9.X none
multiple layers
IMAGE_BINDING_LAYER 8*xZ+ GetIntegeri_v 0 layer of bound texture 3.9.X none
object, if not layered
IMAGE_BINDING_ACCESS 8*xZ3 GetIntegeri_v READ_ONLY read and/or write access 3.9.X none
for bound texture
IMAGE_BINDING_FORMAT 8*xZ+ GetIntegeri_v R8 format used for accesses 3.9.X none
to bound texture
Additions to Appendix A of the OpenGL 3.2 (Compatibility Profile)
Specification (Invariance)
Modify Section A.1, Repeatability (p. 454)
(add a new sentence to the end of the first paragraph, p. 454) ... For
any given GL and framebuffer state vector .. whenever the command is
executed on that initial GL and framebuffer state. This repeatability
requirement doesn't apply when using shaders containing side effects
(image stores, image atomic operations, atomic counter operations),
because these memory operations are not guaranteed to be processed in a
defined order.
Modify Section A.3, Invariance (p. 455)
(add new language to the end of the section, p. 457)
If a sequence of GL commands specifies primitives to be rendered with
shaders containing side effects (image stores, image atomic operations,
atomic counter operations), invariance rules are relaxed. In particular,
Rule 1, Corollary 3, and Rule 4 do not apply in the presence of shader
side effects.
The following weaker versions of Rule 1 and 4 apply to GL commands
involving shader side effects:
Rule 6: For any given GL and framebuffer state vector, and for any
given GL command, the contents of any framebuffer state not directly or
indirectly affected by results of shader image stores, atomic
operations, or atomic counter operations must be identical each time the
command is executed on that initial GL and framebuffer state.
Rule 7: The same vertex or fragment shader will produce the same result
when run multiple times with the same input as long as:
* shader invocations do not use image atomic operations or atomic
counters;
* no framebuffer memory is written to more than once by image stores,
unless all such stores write the same value; and
* no shader invocation, or other operation performed to process the
sequence of commands, reads memory written to by an image store.
When any sequence of GL commands triggers shader invocations that perform
image stores, atomic operations, or atomic counter operations, and
subsequent GL commands read the memory written by those shader
invocations, these operations must be explicitly synchronized. For more
details, see Section 2.14.X, Shader Memory Access.
Modify Section A.3, Invariance (p. 455)
Add Section A.5, Shader Image Load, Store, and Atomic Invariance (p. 457)
Additions to Appendix D of the OpenGL 3.2 (Compatibility Profile)
Specification (Invariance)
Modify Section D.3, Propagating State Changes, p. 467
(add to list of bullets at the end of the section, p. 467)
* Rendering commands that trigger shader invocations, where the shader
performs image stores or atomic operations.
Additions to the AGL/GLX/WGL Specifications
None.
GLX Protocol
!!! TBD !!!
NOTE TO PROTOCOL CREATORS: Don't attempt to use the same protocol for
BindImageTexture and BindImageTextureEXT (from
EXT_shader_image_load_store). BindImageTexture throws an error on
negative and values; BindImageTextureEXT does not.
Modifications to the OpenGL Shading Language Specification, Version 1.50
Including the following line in a shader can be used to control the
language features described in this extension:
#extension GL_ARB_shader_image_load_store :
where is as specified in section 3.3.
New preprocessor #defines are added to the OpenGL Shading Language:
#define GL_ARB_shader_image_load_store 1
Modify Section 3.6, Keywords, p. 14
(add the following to the list of keywords, p. 14)
coherent
volatile
restrict
readonly
writeonly
image1D iimage1D uimage1D
image2D iimage2D uimage2D
image3D iimage3D uimage3D
image2DRect iimage2DRect uimage2DRect
imageCube iimageCube uimageCube
imageBuffer iimageBuffer uimageBuffer
image1DArray iimage1DArray uimage1DArray
image2DArray iimage2DArray uimage2DArray
imageCubeArray iimageCubeArray uimageCubeArray
image2DMS iimage2DMS uimage2DMS
image2DMSArray iimage2DMSArray uimage2DMSArray
(remove from the list of reserved keywords, p. 15)
volatile
Add all these types into the basic types table, in the opaque sections,
along with their corresponding texture types.
(Insert a new section immediately after Section 4.1.7, Samplers, p. 23)
Section 4.1.X, Images
Like samplers, images are opaque handles to one-, two-, or
three-dimensional images corresponding to all or a portion of a single
level of a texture image bound to an image unit. There are distinct
image variable types for each texture target, and for each of float,
integer, and unsigned integer data types. Image accesses should use
an image type that matches the target of the texture whose level is
bound to the image unit, or for non-layered bindings of 3D or array
images should use the image type that matches the dimensionality of
the layer of the image (i.e. a layer of 3D, 2DArray, Cube, or
CubeArray should use image2D, a layer of 1DArray should use image1D,
and a layer of 2DMSArray should use image2DMS). If the image target type
does not match the bound image in this manner, if the data type does not
match the bound image, or if the format layout qualifier does not match
the image unit format as described in Section 3.9.X of the OpenGL
Specification, the results of image accesses are undefined but cannot
include program termination.
Image variables are used in the image load, store, and atomic functions
described in Section 8.X, "Image Functions" to specify an image to access.
They can only be declared as function parameters or uniform variables (see
Section 4.3.5 "Uniform"). Except for array indexing, structure field
selection, and parentheses, images are not allowed to be operands in
expressions. Images may be aggregated into arrays within a shader (using
square brackets [ ]) and can be indexed with general integer expressions.
The results of accessing an image array with an out-of-bounds index are
undefined. Images cannot be treated as l-values; hence, they cannot be
used as out or inout function parameters, nor can they be assigned into.
As uniforms, they are initialized only with the OpenGL API; they cannot be
declared with an initializer in a shader. As function parameters, images
may only be passed to samplers of matching type.
Add Memory Qualifier Table to Section 4.3, Storage Qualifiers, p. 29
Only variables declared as image types (the basic opaque types with
"image" in their keyword) can be qualified with a memory qualifier.
Variables declared as image types can qualified with one or more of the
following memory qualifiers:
Qualifier Meaning
------------ -------------------------------------------------
coherent memory variable where reads and writes are coherent
with reads and writes from other shader invocations
volatile memory variable whose underlying value may be
changed at any point during shader execution by
some source other than the current shader invocation
restrict memory variable where use of that variable is the
only way to read and write the underlying memory
in the relevant shader stage
readonly memory variable that can be used to read the
underlying memory, but cannot be used to write the
underlying memory
writeonly memory variable that can be used to write the
underlying memory, but cannot be used to read the
underlying memory
Modify Section 4.3.2, Constant Qualifier (p. 30)
(add after last paragraph of section)
Because image variables can not be built from constant expressions, the
"const" qualifier may not be used to create a compile-time constant image
variable.
Modify Section 4.3.8.1 (Input Layout Qualifiers), p. 39
Remove "only" from the sentence:
Fragment shaders can have an input layout only for redeclaring the
built-in variable gl_FragCoord...
Add to the end of the section:
Fragment shaders also allow the following layout qualifier on "in" only
(not with variable declarations):
layout-qualifier-id
early_fragment_tests
to request that fragment tests be performed before fragment shader
execution, as described in Section 3.12.2 of the OpenGL Specification.
For example,
layout(early_fragment_tests) in;
Specifying this will make per-fragment tests be performed before fragment
shader execution. If this is not declared, per-fragment tests will be
performed after fragment shader execution.
(Insert immediately after Section 4.3.8.3, Uniform Block Layout
Qualifiers, p. 40)
Section 4.3.8.X, Image Layout Qualifiers
Format layout qualifiers can be used on image variable declarations (those
declared with a basic type having 'image' in its keyword). The format
layout qualifier identifiers for image variable declarations are
:
:
rgba32f
rgba16f
rg32f
rg16f
r11f_g11f_b10f
r32f
r16f
rgba16
rgb10_a2
rgba8
rg16
rg8
r16
r8
rgba16_snorm
rgba8_snorm
rg16_snorm
rg8_snorm
r16_snorm
r8_snorm
:
rgba32i
rgba16i
rgba8i
rg32i
rg16i
rg8i
r32i
r16i
r8i
:
rgba32ui
rgba16ui
rgb10_a2ui
rgba8ui
rg32ui
rg16ui
rg8ui
r32ui
r16ui
r8ui
A format layout qualifier specifies the image format associated with a
declared image variable. Only one format qualifier may be specified for
any image variable declaration. For image variables with floating-point
component types (image*), signed integer component types (iimage*), or
unsigned integer component types (uimage*), the format qualifier used must
match the , , or
grammar rules, respectively. It is an error
to declare an image variable where the format qualifier does not match the
image variable type.
Any image variable used for image loads or atomic operations must specify
a format layout qualifier; it is an error to pass an image uniform
variable or function parameter declared without a format layout qualifier
to an image load or atomic function.
Uniforms not qualified with "writeonly" must have a format layout qualifier.
Note that an image variable passed to a function for read access cannot be
declared as "writeonly" and hence must have been declared with a format
layout qualifier.
(Insert immediately after Section 4.3.9, Interpolation, p. 42)
Section 6.1.1 Function Calling Conventions
Add "memory qualifier" as one of the qualifiers that can be used as a formal
"parameter-qualifier".
Section 4.3.X, Memory Access Qualifiers
The "coherent", "volatile", "restrict", and "const" storage qualifiers can
be specified in image variable declarations to control memory accesses
using the declared variables.
Memory accesses to image variables declared using the "coherent" storage
qualifier are performed coherently with similar accesses from other shader
invocations. In particular, when reading a variable declared as
"coherent", the values returned will reflect the results of previously
completed writes performed by other shader invocations. When writing a
variable declared as "coherent", the values written will be reflected in
subsequent coherent reads performed by other shader invocations. As
described in the Section 2.20.X of the OpenGL Specification, shader memory
reads and writes complete in a largely undefined order. The built-in
function memoryBarrier() can be used if needed to guarantee the completion
and relative ordering of memory accesses performed by a single shader
invocation.
When accessing memory using variables not declared as "coherent", the
memory accessed by a shader may be cached by the implementation to service
future accesses to the same address. Memory stores may be cached in such
a way that the values written may not be visible to other shader
invocations accessing the same memory. The implementation may cache the
values fetched by memory reads and return the same values to any shader
invocation accessing the same memory, even if the underlying memory has
been modified since the first memory read. While variables not declared
as "coherent" may not be useful for communicating between shader
invocations, using non-coherent accesses may result in higher performance.
Memory accesses to image variables declared using the "volatile" storage
qualifier must treat the underlying memory as though it could be read or
written at any point during shader execution by some source other than the
executing shader invocation. When a volatile variable is read, its value
must be re-fetched from the underlying memory, even if the shader
invocation performing the read had previously fetched its value from the
same memory. When a volatile variable is written, its value must be
written to the underlying memory, even if the compiler can conclusively
determine that its value will be overwritten by a subsequent write. Since
the external source reading or writing a "volatile" variable may be
another shader invocation, variables declared as "volatile" are
automatically treated as coherent.
Memory accesses to image variables declared using the "restrict" storage
qualifier may be compiled assuming that the variable used to perform the
memory access is the only way to access the underlying memory using the
shader stage in question. This allows the compiler to coalesce or reorder
loads and stores using "restrict"-qualified image variables in ways that
wouldn't be permitted for image variables not so qualified, because the
compiler can assume that the underlying image won't be read or written by
other code. Applications are responsible for ensuring that image memory
referenced by variables qualified with "restrict" will not be referenced
using other variables in the same scope; otherwise, accesses to
"restrict"-qualified variables will have undefined results.
Memory accesses to image variables declared using the "readonly" qualifier
may only read the underlying memory, which is treated as read-only memory
and cannot be written to. It is an error to pass an image variable qualified
with "readonly" to imageStore() or other built-in functions that modify
image memory.
Memory accesses to image variables declared using the "writeonly" qualifier
may only write the underlying memory; the underlying memory cannot be read.
It is an error to pass an image variable qualified with "writeonly" to
imageLoad() or other built-in functions that read image memory.
The values of image variables qualified with "coherent", "volatile",
"restrict", "readonly", or "writeonly" may not be passed to functions
whose formal parameters lack such qualifiers. (See section 6.1 'Function
Definitions' for more detail on function calling.) It is legal to have
additional qualifiers on a formal parameter, but not to have fewer.
vec4 funcA(layout(rgba32f) image2D restrict a) { ... }
vec4 funcB(layout(rgba32f) image2D a) { ... }
layout(rgba32f) uniform image2D img1;
layout(rgba32f) coherent uniform image2D img2;
funcA(img1); // OK, adding "restrict" is allowed
funcB(img2); // illegal, stripping "coherent" is not
Layout qualifiers cannot be used on formal function parameters, but they are
not included in parameter matching.
Note that the use of "const" in an image variable declaration is qualifying
the const-ness of variable being declared, not the image it refers to: The
qualifier "readonly" qualifies the image memory (as accessed through that
variable) while "const" qualifiers the variable itself.
Modify Section 7.4, Built-In Constants, p. 74
(Add the following new constants.)
const int gl_MaxImageUnits = 8;
const int gl_MaxCombinedImageUnitsAndFragmentOutputs = 8;
const int gl_MaxImageSamples = 0;
const int gl_MaxVertexImageUniforms = 0;
const int gl_MaxTessControlImageUniforms = 0;
const int gl_MaxTessEvaluationImageUniforms = 0;
const int gl_MaxGeometryImageUniforms = 0;
const int gl_MaxFragmentImageUniforms = 8;
const int gl_MaxCombinedImageUniforms = 8;
(Insert a new numbered section at the end of Chapter 8, Built-in
Functions, p. 69)
Section 8.X, Image Functions
Variables using one of the image data types may be used in the built-in
shader image memory functions defined in this section to read and write
individual texels of a texture. Each image variable references an image
unit, which has a texture image attached.
When image memory functions access memory, an individual texel in the
image is identified using an i, (i,j), or (i,j,k) coordinate corresponding
to the values of . For image2DMS and image2DMSArray variables (and
the corresponding int/unsigned int types) corresponding to multisample
textures, each texel may have multiple samples and an individual sample is
identified using the integer parameter. The coordinates and
sample number are used to select an individual texel in the manner
described in Section 3.9.X of the OpenGL specification.
Loads and stores support float, integer, and unsigned integer types. The
data types "gimage*" serve as placeholders meaning either "image*",
"iimage*", or "uimage*" in the same way as "gvec" or "gsampler".
The "IMAGE_INFO" in the prototypes below is a placeholder representing
33 separate functions, each for a different type of image variable. The
"IMAGE_INFO" placeholder is replaced by one of the following parameter
lists:
gimage1D image, int coord
gimage2D image, ivec2 coord
gimage3D image, ivec3 coord
gimage2DRect image, ivec2 coord
gimageCube image, ivec3 coord
gimageBuffer image, int coord
gimage1DArray image, ivec2 coord
gimage2DArray image, ivec3 coord
gimageCubeArray image, ivec3 coord
gimage2DMS image, ivec2 coord, int sample
gimage2DMSArray image, ivec3 coord, int sample
(Note that each of the "gimage*" lines represents one of three different
image variable types.)
Syntax:
gvec4 imageLoad(readonly IMAGE_INFO);
Description:
Loads the texel at the coordinate from the image unit specified
by . For multisample loads, the sample number is given by
. When , , and identify a valid texel,
the bits used to represent the selected texel in memory are converted to
a vec4, ivec4, or uvec4 in the manner described in Section 3.9.X of the
OpenGL Specification and returned.
Syntax:
void imageStore(writeonly IMAGE_INFO, gvec4 data);
Description:
Stores the value of into the texel at the coordinate from
the image specified by . For multisample stores, the sample number
is given by . When , , and identify a
valid texel, the bits used to represent are converted to the format
of the image unit in the manner described in Section 3.9.X of the OpenGL
Specification and stored to the specified texel.
Syntax:
uint imageAtomicAdd(IMAGE_INFO, uint data);
int imageAtomicAdd(IMAGE_INFO, int data);
uint imageAtomicMin(IMAGE_INFO, uint data);
int imageAtomicMin(IMAGE_INFO, int data);
uint imageAtomicMax(IMAGE_INFO, uint data);
int imageAtomicMax(IMAGE_INFO, int data);
uint imageAtomicAnd(IMAGE_INFO, uint data);
int imageAtomicAnd(IMAGE_INFO, int data);
uint imageAtomicOr(IMAGE_INFO, uint data);
int imageAtomicOr(IMAGE_INFO, int data);
uint imageAtomicXor(IMAGE_INFO, uint data);
int imageAtomicXor(IMAGE_INFO, int data);
uint imageAtomicExchange(IMAGE_INFO, uint data);
int imageAtomicExchange(IMAGE_INFO, int data);
uint imageAtomicCompSwap(IMAGE_INFO, uint compare, uint data);
int imageAtomicCompSwap(IMAGE_INFO, int compare, int data);
Description:
These functions perform atomic operations on individual texels or samples
of an image variable. Atomic memory operations read a value from the
selected texel, compute a new value using one of the operations described
below, write the new value to the selected texel, and return the
original value read. The contents of the texel being updated by the
atomic operation are guaranteed not to be updated by any other image store
or atomic function between the time the original value is read and the
time the new value is written.
As with image load and store functions, , , and
specify the individual texel to operate on. The method for
identifying the individual texel operated on from , , and
, and the method for reading and writing the texel are specified
in Section 3.9.X of the OpenGL specification. Atomic memory operations
are supported on only a subset of all image variable types; must
be either:
* an image variable with signed integer components (iimage*) and a
format qualifier of "r32i", or
* an image variable with unsigned integer components (uimage*) and a
format qualifier of "r32ui".
imageAtomicAdd() computes a new value by adding the value of to the
contents of the selected texel. These functions support 32-bit unsigned
integer operands and 32-bit signed integer operands.
imageAtomicMin() computes a new value by taking the minimum of the value
of and the contents of the selected texel. These functions support
32-bit signed and unsigned integer operands.
imageAtomicMax() computes a new value by taking the maximum of the value
of and the contents of the selected texel. These functions support
32-bit signed and unsigned integer operands.
imageAtomicAnd() computes a new value by performing a bitwise and of the
value of and the contents of the selected texel. These functions
support 32-bit signed and unsigned integer operands.
imageAtomicOr() computes a new value by performing a bitwise or of the
value of and the contents of the selected texel. These functions
support 32-bit signed and unsigned integer operands.
imageAtomicXor() computes a new value by performing a bitwise exclusive or
of the value of and the contents of the selected texel. These
functions support 32-bit signed and unsigned integer operands.
imageAtomicExchange() computes a new value by simply copying the value of
. These functions support 32-bit signed and unsigned integer
operands.
imageAtomicCompSwap() compares the value of and the contents of
the selected texel. If the values are equal, the new value is given by
; otherwise, it is taken from the original value loaded from the
texel. These functions support 32-bit signed and unsigned integer
operands.
(Insert another new numbered section at the end of Chapter 8, Built-in
Functions, p. 69)
Section 8.Y, Shader Memory Control Functions
Shaders of all types may read and write the contents of textures and
buffer objects using image variables. While the order of reads and writes
visible to a single shader invocation is well-defined, the relative order
of reads and writes to a single shared memory address from multiple
separate shader invocations is largely undefined. Additionally, the order
of accesses to multiple memory addresses performed by a single shader
invocation, as observed by other shader invocations, is also undefined.
Syntax:
void memoryBarrier(void);
Description:
memoryBarrier() can be used to control the ordering of memory transactions
issued by a single shader invocation. When called, memoryBarrier() will
wait on the completion of all memory accesses resulting from the use of
image variables or atomic counters and then return to the caller with no
other effect. When this function returns, the results of any memory
stores performed using coherent variables performed prior to the call will
be visible to any future coherent memory access to the same addresses from
other shader invocations. In particular, the values written this way in
one shader stage are guaranteed to be visible to coherent memory accesses
performed by shader invocations in subsequent stages when those
invocations were triggered by the execution of the original shader
invocation (e.g., fragment shader invocations for a primitive resulting
from a particular geometry shader invocation).
Modify Section 9, Shading Language Grammar (p. 105)
!!! TBD: Add grammar constructs for memory access qualifiers.
Errors
INVALID_VALUE is generated by Uniform1i{v} if the location refers to an
image variable and the value specified is less than zero or greater than
or equal to the value of MAX_IMAGE_UNITS.
INVALID_OPERATION is generated by Uniform* functions other than
Uniform1i{v} if the location refers to an image variable.
INVALID_VALUE is generated by BindImageTexture if is greater
than or equal to the value of MAX_IMAGE_UNITS.
INVALID_VALUE is generated by BindImageTexture if is not the
name of an existing texture object.
INVALID_VALUE is generated by BindImageTexture if is not a
legal format.
Dependencies on OpenGL 3.2 (Core Profile)
If only the core profile of OpenGL 3.2 is supported, references to buffer
objects for conventional vertex attributes and to the Begin and RasterPos
commands should be removed.
Dependencies on OpenGL 3.1, ARB_uniform_buffer_object, and
EXT_bindable_uniform
If OpenGL 3.1, ARB_uniform_buffer_object, and EXT_bindable_uniform are not
supported, references to UNIFORM_BARRIER_BIT should be removed.
Dependencies on ARB_draw_indirect
If ARB_draw_indirect is not supported, references to COMMAND_BARRIER_BIT
should be removed.
Dependencies on NV_vertex_buffer_unified_memory
If NV_vertex_buffer_unified_memory is not supported, references to that
extension and GPU addresses in the discussion of
VERTEX_ATTRIB_ARRAY_BARRIER_BIT and ELEMENT_ARRAY_BARRIER_BIT should
be removed.
Dependencies on NV_parameter_buffer_object
If NV_parameter_buffer_object is not supported, references to
ProgramBufferParametersNV in the discussion of BUFFER_UPDATE_PARAMETER_BIT
should be removed.
Dependencies on OpenGL 3.2 and ARB_texture_multisample
If OpenGL 3.2 and ARB_texture_multisample are not supported, references to
multisample textures should be removed.
Dependencies on OpenGL 4.0 and ARB_sample_shading
If OpenGL 4.0 or ARB_sample_shading is supported, the discussion of the
number of shader invocations for a given fragment in the "Shader Memory
Access" section of the specification should be updated to discuss the
sample shading enable and the minimum sample shading factor provided in
that extension.
Dependencies on OpenGL 4.0 and ARB_texture_cube_map_array
If OpenGL 4.0 or ARB_texture_cube_map_array are not supported, references
to cube map array textures should be removed.
Dependencies on OpenGL 3.3 and ARB_texture_rgb10_a2ui
If OpenGL 3.3 or ARB_texture_rgb10_a2ui are not supported, references to
the RGB10_A2UI texture format should be removed.
Dependencies on NV_shader_buffer_load
If NV_shader_buffer_load is supported, the new section 2.14.X (Shader
Memory Access) should be combined with "Section 2.20.X, Shader Memory
Access" from NV_shader_buffer_load.
Dependencies on OpenGL 4.0, ARB_gpu_shader5, and NV_gpu_shader5
If OpenGL 4.0, ARB_gpu_shader5, and NV_gpu_shader5 are not supported, the
modifications to the OpenGL Shading Language Specification should be
removed.
Dependencies on OpenGL 4.0 and ARB_tessellation_shader
If OpenGL 4.0 and ARB_tessellation_shader are not supported, references to
tessellation control and evaluation shaders should be removed.
Dependencies on EXT_shader_atomic_counters and ARB_shader_atomic_counters
If EXT_shader_atomic_counters is not supported, remove references to
atomic counters and ATOMIC_COUNTER_BARRIER_BIT.
Dependencies on EXT_depth_bounds_test
If EXT_depth_bounds_test is not supported, references to the depth bounds
test should be removed.
Dependencies on ARB_separate_shader_objects
If ARB_separate_shader_objects is supported, early depth tests are enabled
if and only if (a) there is an active program for the fragment shader
stage and (b) the fragment shader in that program enables early depth
tests using a layout qualifier.
Dependencies on EXT_shader_image_load_store
Both this extension and EXT_shader_image_load_store provide nearly the
identical functionality.
If both extensions are enabled in the shading language, the "size*" layout
qualifiers are treated as format qualifiers, and are mapped to equivalent
format qualifiers in the table below, according to the type of image
variable. Additionally, if both extensions are enabled in the shading
language, size/format layout qualifiers need not be specified for image
variables used exclusively for stores.
image* iimage* uimage*
-------- -------- --------
size1x8 n/a r8i r8ui
size1x16 r16f r16i r16ui
size1x32 r32f r32i r32ui
size2x32 rg32f rg32i rg32ui
size4x32 rgba32f rgba32i rgba32ui
Issues
(0) How does this extension differ from the similar
EXT_shader_image_load_store?
RESOLVED: The functionality provided by this extension is very similar
to that provided by EXT_shader_image_load_stores. There are some
functional differences.
* "size" layout qualifiers replaced with "format" qualifiers.
* Image loads aren't restricted to "1x8", "1x16", "1x32", "2x32", and
"4x32" formats. Instead, each supported image format has a layout
qualifier, and values loaded from images are converted to an
vec4/ivec4/uvec4 representation appropriate for the image format.
* For textures not allocated by the GL (e.g., images shared from other
external APIs), implementations need not support image unit formats
that don't match the texture format, unless they are in the same
"class", which is generally the case only if component counts and
sizes are exactly the same.
* Image variables used exclusively for image stores need not declare a
format qualifier.
* Added the built-in GLSL constants "gl_MaxImageUnits",
"gl_MaxCombinedImageUnitsAndFragmentOutputs", and
"gl_MaxImageSamples".
* BindImageTexture throws INVALID_VALUE if or is
negative.
* The parameter of BindImageTexture was changed from an "int"
to an "enum". In the EXT, copied TexImage*'s
parameter, which is an "int" because that's how it
was defined in OpenGL 1.0 (where the parameter was called
and the now-deprecated "1", "2", "3", and "4" formats
were the only ones supported).
* Added implemenentation-dependent limits on the number of active
image uniforms (MAX_*_IMAGE_UNIFORMS) for each stage, and combined
across all stages. Also added corresponding GLSL constants
"gl_Max*ImageUniforms".
* The atomicIncWrap() and atomicDecWrap() built-in functions present
in the EXT have been removed.
* The parameter of BindImageTextureEXT has been renamed to
for BindImageTexture.
(1) How are the format and type of the load/store determined?
RESOLVED: There is a natural desire to load and store using a
canonical 4-vector in the shader with hardware converting to/from a
format compatible with the bound image, to be consistent with how
texture loads and fragment shader outputs currently behave. There is
also good reason to allow some flexibility in the format used for image
accesses being different from the internal format of the texture level.
We allow format conversions to and from any format that image units
support. We make the format be selected when the image is bound to an
image unit, and define which image unit formats can be used for which
texture level internal formats. For example, it is legal to access an
image whose internal format is RGBA8 with an image unit format of
R32UI.
(2) What set of texture formats should be supported for image loads and
stores?
RESOLVED: We allow textures to be bound to image units using only a
subset of supported formats, to limit the amount of hardware support
required for image operations. Any texture formats not explicitly
enumerated in this extension may not be bound to an image unit, although
future extensions may add new formats to the set of supported formats.
In particular, this extension supports one-, two-, and four-component
textures with 8-, 16-, and 32-bit components, including floating-point,
signed integer, unsigned integer, as well as signed and unsigned
normalized formats. Additionally, a small number of other formats are
supported, including the 11/11/10 RGB format from EXT_packed_float and
10/10/10/2 unsigned normalized RGBA.
(3) Should we general support image loads and stores for three-component
"RGB" formats?
RESOLVED: Not in this extension. If an application needs to perform
image loads and stores on a three-component texture, it could use an
equivalent RGBA format and ignore the alpha component. The
EXT_texture_swizzle extension could be used to make the values returned
by texture appear identical to an RGB texture, if required.
(4) Should textures be unbound from image units when they are deleted?
RESOLVED: Yes, this matches behavior of existing bind points.
(5) Should we support image loads and stores for the deprecated LUMINANCE,
LUMINANCE_ALPHA, and ALPHA formats?
RESOLVED: No, only support the RGBA-style formats. EXT_texture_swizzle
can be used to mimic luminance and alpha if required.
(6) Should we support 64-bit atomics on images? Should we support atomics
at all on formats with 8-, 16-, 64-, or 128-bit texels?
RESOLVED: No, we will only support 32-bit atomic operations on images.
(7) How do shader image loads and stores interact with texture
completeness? What happens if you bind a texture with inconsistent
mipmaps?
RESOLVED: The image unit is treated as if nothing were bound, where
all accesses are treated as invalid.
(8) What happens if the value passed to Uniform1i to specify the image
unit corresponding to a image variable refers to a non-existent image
unit (i.e., is negative or greater than or equal to the number of
image units supported)?
RESOLVED: Values referring to invalid image units will be rejected and
produce an INVALID_VALUE error.
(9) Should we provide counting rules for image variable use in different
shaders like we have for samplers? In particular, there are limits
on the amount of state, the number of active samplers in each shader
stage, and the sum of the active sampler counts in each stage.
RESOLVED: Yes, we provide a similar set of limits. MAX_IMAGE_UNITS
specifies the number of image bindings. MAX_{VERTEX,...}_IMAGE_UNIFORMS
specifies the maximum number of active image uniforms in each shader
stage. MAX_COMBINED_IMAGE_UNIFORMS specifies a limit on the sum of the
number of active image uniforms in all stages of a program.
(10) Can this extension be used to load and store values into a buffer
object? Into a renderbuffer?
RESOLVED: Yes, indirectly. The BUFFER_TEXTURE target provided by
OpenGL 3.0 and the EXT_texture_buffer_object extension allows an
application to create a one-dimensional buffer texture using the data
store of a buffer object. This buffer texture may be bound to an image
unit and accessed with an imageBuffer variable in the Shading Language.
This extension adds support for image accesses to multisample textures,
but not renderbuffers. Note that with the ARB_texture_multisample
extension, there is no longer a good reason to use renderbuffers.
Existing 2D or rectangle targets already provided a superset of single-
sample renderbuffer functionality; the new ARB extension provides a
superset of multisample renderbuffer functionality.
(11) What amount of automatic synchronization is provided for image loads
and stores? In particular, is the use of MemoryBarrier() required
to ensure consistent ordering relative to other GL operations? Or is
some other mechanism (e.g., unbinding a texture from an image unit
and then binding it to a texture image unit) sufficient?
RESOLVED: Use of MemoryBarrier is required, and there is no
automatic synchronization when images are bound or unbound.
Implicit synchronization is difficult, as it might require some
combination of:
- tracking which images might be written (randomly) in the shader
itself;
- assuming that if a shader that performs writes is executed, all
texels of all bound images could be modified and thus must be
treated as dirty;
- idling at the end of each primitive or draw call, so that the
results of all previous commands are complete.
Since normal OpenGL operation is pipelined, idling would result in a
significant performance impact since pipelining would otherwise allow
fragment shader execution for draw call N while simultaneously
performing vertex shader execution for draw call N+1.
(12) Should image loads and stores be allowed for all shader types?
RESOLVED: Yes, it seems useful.
Note that some shader types pose specific implementation complexities
(e.g., reuse of vertices in vertex shaders, number of fragment shader
invocations in multisample modes, relative order of execution within and
between shader groups). We have explicitly specify several cases where
the invocation count and execution order are undefined. While these
cases may be a problem for some algorithms, we expect that many
algorithms will not be adversely impacted.
(13) Should an implementation be required to throw INVALID_OPERATION
errors if the dimension of the texture coordinates implied by the
image variable type doesn't match the structure of the texture
level/layer bound to the corresponding image unit? If not, what
happens in such a mismatch?
RESOLVED: No. The results of image accesses are undefined.
(14) Should shader image variable types include a "format" implying the
data type accepted/returned by shader image loads and stores? For
example, an image variable corresponding to a 2D texture with format
of RGBA32F might have a type "image2Dvec4", with the "vec4"
indicating that the image data lines up with a four-component
floating-point vector.
RESOLVED: No. Separate types are provided for float vs. int vs.
unsigned int, but not for each image format. However, format qualifiers
associated with image variables can (and in many cases must) be used to
associate a format with an image variable.
(15) If shader image variable types include information on the texel
components returned or written by shader image accesses, should an
implementation be required to enforce errors if the variable type is
incompatible with the format of the referenced texture? If not, or
if the image variable type doesn't include format information, what
happens in case of a mismatch between the texture format and the
shader access format?
RESOLVED: We aren't including types in the variable that correspond
to the image format, so an error check in the driver is not possible.
If an individual load, store, or atomic uses a data type incompatible
with the texture bound to the image unit, loads will return and stores
will write undefined values.
(16) Is it possible to bind the "default texture" (numbered zero) for a
given texture target to an image unit?
RESOLVED: No. Passing zero to BindImageTexture unbinds and texture
currently bound to the selected image unit. If this ability were
provided, it would also be necessary to provide some mechanism to
specify a texture target because there is a separate default "zero"
texture for each target.
Note that existing framebuffer objects have a similar behavior; default
textures can't be attached to an FBO.
(17) May bordered textures be used with image loads and stores?
RESOLVED: No.
(18) Should we have defined behavior if invalid coordinates are passed to
an image load, store, or atomic operation? If so, what happens?
RESOLVED: Yes. We define the behavior to return zeroes on a load and
atomic and to have no effect on any bound texture on stores and
atomics.
(19) Should we have a limit on the total number of combined image units
and draw buffers, and if so, what should that be?
RESOLVED: Yes, some hardware requires this. The program will fail to
link.
(20) What happens if a shader specifies an image store or atomic operation
for killed/discarded pixels?
RESOLVED: For GLSL shaders that execute a "discard" instruction, any
image stores or atomics performed before executing the discard will
behave normally. When the "discard" instruction is executed, the shader
invocation will be terminated and will perform no further image store or
atomic operations.
(21) When enabling early depth tests in a program, what happens if a
fragment fails one of the tests (e.g., depth test)?
RESOLVED: The specification indicates that the fragment shader is not
executed. Implementations might still end up running fragment shader
for implementation-dependent reasons. For example, the fragment shader
may be run in order to approximate derivatives for neighboring pixels
that did pass all per-fragment tests. In these cases, implementations
must guarantee that image stores have no effect.
(22) If implementations run fragment shaders for fragments that aren't
covered by the primitive or fail early depth tests (e.g., "helper
pixels"), how does that interact with stores and atomics?
RESOLVED: The current OpenGL specification has no formal notion of
"helper" pixels. In practice, implementations may run fragment shaders
for pixels near the boundaries of rasterized primitives to allow
derivatives to be approximated by differencing. Typically, these shader
invocations have no effect. While they may produce outputs, the outputs
for these pixels will be discarded without affecting the framebuffer.
The spec basically treats these shader invocations as though they don't
exist.
If such a shader invocation performs store or atomic operations, we need
to define what happens. In our definition, stores will have no effect,
atomics will not update memory, and the values returned by atomics will
be undefined. The fact that these invocations don't affect memory is
consistent with the notion of helper pixel shader invocations not
existing.
However, it is possible to write a fragment shader where flow control
depends on the (undefined) values returned by the atomic. In this case,
the undefined values returned for helper pixels could result in very
long execution time (appearing to be hang) or an infinite loop. To
avoid hangs in such cases, it is possible to use the fragment shader
input sample mask to identify helper pixels:
// If the input sample mask is non-zero, at least one sample is
// covered and the invocation should be treated as a real invocation.
// If the sample mask is zero, nothing is covered and this should be
// treated as a helper pixel. If more than 32 samples are supported,
// additional words of gl_SampleMaskIn would need to be checked.
if (gl_SampleMaskIn[0] != 0) {
// "real" pixel, perform atomic operations
} else {
// "helper" pixel, skip atomics
}
It may be desirable to formalize the notion of helper pixels in a future
addition to the shading language.
(23) What API should we use to specify early depth tests?
RESOLVED: Use a layout qualifier in a fragment shader rather than
having a separate program parameter or other piece of GL state.
(24) For formatted loads where the format doesn't include some component,
what values are filled in? (0,0,0,1)? (0,0,0,0)?
RESOLVED: Prefer (0,0,0,1) to match other APIs.
(25) How does the combined-image-and-fragment-output limit interact with
separate shader objects? For example, an application may want to
share a single image unit between two shader stages and not have it
count twice against the limit.
RESOLVED: The known implementations of this extension do not have this
issue, so we chose not to include any spec language. Perhaps a
Begin-time error could be specified in the future if this limit is
exceeded.
(26) What sort of qualifiers should we provide relevant to memory
referenced by image variables?
RESOLVED: We will support the qualifiers "coherent", "volatile",
"restrict", and "const" to be used in image variable declarations.
"coherent" is used to ensure that memory accesses from different shader
invocations are cached coherently (i.e., one invocation will be able to
observe writes from another when the other invocation's writes
complete). This coherence may mean the use of "coherent"-qualified
image variables may perform more slowly than of otherwise equivalent
unqualified variables.
"volatile" behaves as in C, and may be needed if an algorithm requires
reading image memory that may be written asynchronously by other shader
invocations.
"restrict" behaves as in the C99 standard, and can be used to indicate
that no other image variable points to the same underlying data. This
permits optimizations that would otherwise be impossible if the compiler
has to assume that a pair of images might end up pointing to the same
data. For example, in standard C/C++, a loop like:
int *a, *b;
a[0] = b[0] + b[0];
a[1] = b[0] + b[1];
a[2] = b[0] + b[2];
would need to reload b[0] for each assignment because a[0] or a[1] might
point at the same data as b[0]. With restrict, the compiler can assume
that b[0] is not modified by any of the instructions and load it just
once. The same considerations apply to accesses using imageLoad(),
imageStore(), and imageAtomic*() builtins.
"const" behaves as in C, and indicates that the image memory should be
treated as read-only. Note that the use of "const" in image variable
declarations is different from the normal "const" qualifier, as it
treats the image data referenced by the variable as constant.
(27) How should shaders be able to express qualifiers for image variables?
RESOLVED: This extension borrows from C/C++ syntax rules where a
qualifier may be specified before or after the type. For example,
layout(rgba32f) const uniform image2D imageVariable;
declare an image uniform whose image data are treated as read-only. We
permit qualifiers to be provided either before or after the type name
(image2D). The position of the qualifier is meaningful. Qualifiers
before the type name apply to the data referenced by the variable.
Qualifiers after the type name apply to the variable itself.
The closest C/C++ equivalent to the declarations above would turn
declarations like:
layout(rgba32f) const uniform image2D firstImage;
layout(rgba32f) uniform image2D const secondImage;
into:
const struct image2D_data * firstImage;
struct image2D_data * const secondImage;
where "image2D" is replaced with "struct image2D_data *". In this
model, the former declares to be a pointer to constant
image data. The latter declares to be a constant pointer
to non-constant image data.
For "coherent", "volatile", and "const", the qualifier should typically
go before the image type. For "restrict", the qualifier must go after
the image type, since "restrict" applies to the pointer, not the data
being pointed to.
Note that a qualifier could theoretically be specified before and after
the type name, such as:
const image2D const imageVariable;
which would declare to be constant and to reference
constant image data. In this extension, declaring an image variable to
be constant isn't meaningful, as such variables can never be used as
l-values.
(28) What is the meaning of "restrict" on a system that might run either
multiple invocations of the same shader simultaneously, or multiple
invocations of different shaders (vertex and fragment)
simultaneously?
RESOLVED: When an image variable is qualified with "restrict", the only
guarantee is that no other image variable in the same shader invocation
references the same underlying image data. There is no guarantee that
the same image couldn't be referenced by another invocation of the same
shader, or by an invocation of a different shader.
The main function of "restrict" is to allow compilers to generate more
efficient code for a single shader invocation than it could if it had to
conservatively assume that accesses to other images could touch the same
image data.
(29) What is the purpose of the memoryBarrier() built-in function?
RESOLVED: The memoryBarrier() function can be used to ensure that if
another shader invocation or other portions observe image memory being
written by a shader, that accesses appear in a predictable order. For
example, consider the following code:
uniform imageBuffer buf1;
uniform imageBuffer buf2;
int offset1, offset2;
vec4 data1, data2;
imageStore(buf1, offset1, data1);
imageStore(buf2, offset2, data2);
This specification doesn't require that writes be committed to memory in
the order specified in the shader. It is possible that another shader
invocation or some other observer would see before seeing
. If an algorithm involved multiple shader invocations with one
possibly needing to wait on data written by another, observing
in the second shader would not ensure that has been written.
However, if memoryBarrier() were used, as in the following code, the
second shader would have such a guarantee.
imageStore(buf1, offset1, data1);
memoryBarrier();
imageStore(buf2, offset2, data2);
(30) What happens if the texel identified by the coordinates given to an
image load, store, or atomic built-in doesn't exist? (i.e.,
coordinates are out of bounds)
RESOLVED: The results of image loads return zero. Stores do not update
image memory. Atomics do not update image memory and return zero.
These same considerations apply if no texture is bound to an image unit,
the texture is incomplete, and various other conditions. We do not ever
apply wrap modes on image operations.
(31) Why do we have a parameter on BindImageTexture?
RESOLVED: It allows some amount of bit-casting, to view a texture with
one format using another format. In addition to any benefits from
viewing textures with a different format, it also permits atomics
operations on some multi-component textures by allowing them to be
viewed using R32I or R32UI formats.
In the EXT_shader_image_load_store extension, there was an additional
benefit to working around a more severe limitation on the set of formats
supported for stores -- only formats like R8, R16, R32F, RG32F, RGBA32F
are supported there. Other formats not supported there can be viewed as
supported formats (e.g., RGBA8 could map to R32UI), with shader code
doing any needed packing and unpacking.
(32) Do we support image atomics on multi-component texture formats?
RESOLVED: Only if the texture formats can be viewed as "R32I" or
"R32UI" formats by using the parameter of BindImageTexture.
Atomics do not operate on a component-by-component basis in this
extension.
(33) What happens if early fragment testing is enabled, the early depth
test passes, and a fragment shader that computes a new depth value is
executed?
RESOLVED: The depth value produced by the fragment shader has no effect
if early depth and stencil tests are enabled. The depth value computed
by a fragment shader is used only by the post-fragment shader stencil
and depth tests, and those tests always have no effect when early
fragment tests is enabled.
(34) How do early fragment tests interact with occlusion queries?
RESOLVED: When early fragment tests are enabled, sample counting for
occlusion queries also happens prior to fragment shader execution.
Enabling early fragment tests can change the overall sample count,
because samples killed by alpha test and alpha to coverage will still be
counted if early fragment tests are enabled.
(35) If we provide support for multiple active program objects (e.g., one
containing a vertex shader, another containing a fragment shader, as
in EXT_separate_shader_object), how will early fragment tests be
handled?
RESOLVED: The early fragment test enable should be taken from the
active program object corresponding to the fragment shader stage.
(36) When specifying a coordinate vector to specify a texel for a
TEXTURE_1D_ARRAY target, what coordinate is used to specify the
layer?
RESOLVED: For GLSL functions, a two-component vector is specified and
the second (y) component is used to select a layer.
(37) How does the synchronization (or lack thereof) of shader accesses to
buffer memory interact with accesses to mapped buffer memory?
RESOLVED: Shader memory accesses are not automatically synchronized
with MapBuffer. Mapping a buffer object will not guarantee that image
stores or atomics issued by shaders triggered by rendering commands
prior to the MapBuffer call are complete before returning a pointer to
the application. This lack of synchronization is similar to what
happens if you call MapBufferRange with MAP_UNSYNCHRONIZED_BIT set.
However, if you call MemoryBarrier with BUFFER_UPDATE_BARRIER_BIT set
prior to mapping the buffer object, the GL will manually synchronize,
ensuring that all prior shader writes to a buffer are complete prior to
any subsequent commands (including MapBuffer) accessing the buffer.
Revision History
Rev. Date Author Changes
---- -------- -------- -----------------------------------------------
36 10/27/14 pbrown Fix the "Name Strings" entry to include a "GL_"
prefix.
35 09/11/14 pbrown Add missing text for issue (9).
34 06/10/14 Jon Leech Minor typo fixes from bug 7263.
33 10/16/13 pbrown Update issue (20) to clarify that any image
stores and atomics issued before a "discard" do
have an effect. Update issue (22) to better
define the behavior of stores and atomics on
"helper" pixels and to suggest a workaround for
shaders that need to use values returned by
atomics (undefined for helper pixels) in flow
control constructs.
have an effect.
32 03/06/12 pbrown Fix the minimum values for GLSL built-ins
gl_Max{Fragment,Combined}ImageUniforms to 8
to match the minimums for the API specification
(bug 8673).
31 01/18/12 Jon Leech State table fix for
IMAGE_FORMAT_COMPATIBILITY_TYPE (Bug 8430).
30 08/04/11 pbrown Remove imageAtomicIncWrap() and
imageAtomicDecWrap() functions from the ARB
extension (bug 7182). Rename the
argument of BindImageTexture to (bug
7851). Fix typo in spec language describing
out-of-bounds indexing of image arrays.
29 08/03/11 pbrown Clarify that negative values of and
will generate errors even in cases where
those parameters would ultimately have no effect
(bug 7850). Add a note recommending that
BindImageTexture not use the same protocol as
its "EXT" equivalent due to differences in error
behavior.
28 07/27/11 pbrown Document new implementation limits added in
version 26 as differences from the EXT (bug 7805).
27 07/22/11 Jon Leech Remove unreachable error condition for negative
(bug 7770).
26 07/22/11 pbrown Add implementation limits on the number of
image uniforms used by each shader stage, and on
the combined total of all stages, as well as
corresponding GLSL constants (bug 7805).
25 06/20/11 johnk Sync. with core specification: adds writeonly,
replaces "const" with "readonly", refers to all
these as "memory qualifiers", includes semantics
for calling functions. Minor non-functional
edits to make them match.
24 06/19/11 pbrown Assign values for new enumerants.
23 06/18/11 pbrown Clarify that image variables can not be stored
in uniform blocks.
22 06/07/11 pbrown Clarify that and are ignored
when used with non-layered textures. Clarify
that non-existent layer/face numbers make
accesses invalid for both non-layered and
layered bindings (bug 7721).
21 06/06/11 pbrown Add IMAGE_FORMAT_COMPATIBILITY_TYPE to state
tables, add descriptions for IMAGE_BINDING*
state table entries (bug 7689).
20 06/06/11 pbrown Clarify the language describing data type
conversions for image loads/stores to indicate
that we use the same general process as for
TexImage and GetTexImage commands. For
multisample textures used as images, TexImage
commands do not specify image data and
GetTexImage is not supported (bug 7249).
19 02/14/11 pbrown Remove the repeatability requirement (Appendix
A.1) when using shaders with side effects (bug
7026). Clean up spec language describing GLSL's
memoryBarrier() function, and add a dependency
on atomic counter extensions to indicate that
memoryBarrier() also applies to atomic counters
(bug 7237).
18 02/13/11 Jon Leech Cleanup BindImageTexture language to match
4.2 core spec phrasing.
17 01/20/11 pbrown Clarify that the MAX* limits can be queried
by GetInteger64v (bug 7225). Add INVALID_VALUE
error for BindImageTexture if or
is negative (bug 7226). Clarify language for
MemoryBarrier's BUFFER_UPDATE_BARRIER_BIT (bug
7228). Update the parameter of
BindImageTexture to be an "enum" instead of
"int". The EXT used "int" to be compatible
with TexImage, which itself derived to the
deprecated "1", "2", "3", and "4" formats from
OpenGL 1.0 (bug 7183).
16 01/20/11 pbrown Add GLSL built-in constants for implementation-
dependent limits (bug 7234).
15 01/18/11 Jon Leech Fix typos from Bug 7235.
14 01/18/11 pbrown Add interaction with NV_parameter_buffer_object
for ProgramBufferParametersNV (bug 7235).
13 01/05/11 Jon Leech Fix typos from Bug 7202.
12 12/17/10 johnk Minor tweaks for grammar and consistency that
also apply to 1.5, that were generated while
incorporating into 4.2 core.
11 12/14/10 pbrown Add edits to invariance and synchronization
rules in Appendices A and D to account for
side effects from shader execution (bug 7026).
10 12/14/10 pbrown Clean up issues section from changes in
revisions 7-9.
9 12/14/10 pbrown Modify the layout qualifier behavior for image
variables to specify a full GL-style format
instead of component/bit counts (bug 6868),
with loaded data converted to a canonical
vector type according to the full format.
Limited the amount of format mismatching allowed
when binding textures allocated outside the GL
to image units. Removed the requirement for
layout qualifiers on image variables used
only for image stores.
8 12/12/10 pbrown Additional minor spec errata fixes (bug 6991).
7 12/12/10 pbrown Fix minor spec errata (bug 6870). Removed
interactions with NV_gpu_program5; this is
already covered by the EXT version of the spec.
6 10/19/10 pdaniell ARBify in preparation for OpenGL 4.2 core.
5 09/17/10 pbrown Clean up the spec language specifying the
mapping of coordinates to texels according to
the texture target. For 1D arrays, GLSL wants
the layer in the second component of a
two-component vector while NV_gpu_program5 wants
it in the third component of a four-component
vector. Also clarify that single-layer bindings
of an array or cube map texture use a target
appropriate to the bound layer.
4 03/23/10 pbrown Add interaction with EXT_separate_shader_objects.
Update issues section to include some issues
left behind in NV_gpu_shader5 when specs were
refactored.
3 03/21/10 pbrown Update spec overview, interactions, and issues
sections; miscellaneous minor clarifications.
2 03/16/10 pbrown Add a separate #extension line for this
extension; needed since the became packaged
separately from ARB_gpu_shader5. Added C99-like
"restrict" qualifier to indicate that an image
variable won't share underlying image contents
with any other variable. Added support for
"const" qualifiers on images to allow indicate
read-only image data. Added language describing
the significance of the position of image
variable qualifiers. Clarified rules on use of
image variables as function parameters; adding
qualifiers is OK, stripping them off is not.
Updated image layout qualifier section to
clarify that "size" layout qualifiers are
required on both uniform and function parameter
declarations. Added "const" qualifier on the
image argument in imageLoad() prototypes.
Updated extension names in dependency sections.
Add support for stores to the RGB10_A2 texture
format from OpenGL 3.3. Add several issues.
1 jbolz Internal revisions.